1. Field of the Invention
This invention relates to hydrophones for use under great hydrostatic pressures and under conditions of severe hydrodynamic stress.
2. Discussion of the Prior Art
Piezoelectric hydrophones of various configurations are well known for use in seismic exploration, for tracking submarines in the deep ocean, for vertical seismic surveys in bore holes and for many other uses. Variations in the acoustic pressure produce an electrical output from the hydrophones measured in microvolts to millivolts.
Typically, commercial piezoelectric hydrophones employ ceramic materials such as barium titanate or lead zirconate titanate. The active element of the hydrophone may take the form of a thin wafer that operates in the bender mode as exemplified by U.S. Pat. No. 4,092,628. Other units may be shaped as a right cylinder that operates in the radial mode such as shown by U.S. Pat. No. 4,525,645. The ceramic material is brittle. In the presence of a severe shock wave, such as from a nearby explosive charge or from a powerful air gun, the piezoelectric element shatters. Most hydrophones have a definite depth limit. An excessive overpressure causes the element to bend beyond its elastic limit, resulting in signal distortion and ultimate failure of the instrument. In the case of the wafer configuration, an internal stop may be provided to prevent excessive bending but the wafer tends to develop a permanent set that degrades the output signal.
Another class of material is polyvinylidene fluoride (PVDF), a piezoelectric polymer film, such as KYNAR Piezo film made by Pennwalt Corp. of Valley Forge, Pa. The film is useful as a hydrophone element because its acoustic impedance is close to that of water; acoustic wavefields do not suffer spurious reflections and diffractions as they do when encountering ceramic piezoelectric elements. The signal output is many times higher than the signal output of ceramic devices. Available in sheets of any size and a wide range in thickness, piezoelectric film may be readily shaped and cut to fit the intended use. Prior to use, the material is poled or activated in the thickness direction by application of a high electric field at an elevated temperature for a requisite period of time. Conductive-metal electrodes are evaporated on the opposite sides of the film as with ceramic materials.
An external mechanical force applied to the film results in a compressive or tensile strain. The film develops a proportionate open circuit voltage, that is, an electrical charge proportional to changes in mechanical stress or strain. The charge developed diminishes with time, depending on the dielectric constant of the film and the impedance of the connected circuitry. By convention, the polarization axis is the thickness axis. Tensile stress may take place along either the longitudinal axis or the width axis.
One type of transducer, disclosed in U.S. Pat. No. 4,653,036, employs a PVDF membrane stretched over a hoop ring. A metallic backing is attached to the back of the hoop ring and the void between the film and the backing is filled with an elastomer such as silicone. The device appears to operate in the bender mode. U.S. Pat. No. 4,789,971 teaches use of a voided slab of PVDF material sandwiched between a pair of copper electrodes. A bilaminar construction is also disclosed. A preamplifier is included in the assembly. The transducer operates in the thickness-compressive mode.
A hydrophone array shown in U.S. Pat. No. 4,805,157 consists of multiple layers of PVDF material symmetrically disposed around a stiffener for prevention of flexural stresses. The axis of maximum sensitivity is in the direction transverse to the plane of the layers. It is therefore sensitive to compressive stresses.
In this disclosure, the term "hydrostatic pressure" refers to the static pressure due to a column of water at some particular depth. The term "hydrodynamic pressure" refers to the dynamic pressure differences that are created by acoustic wave fields propagating through the water at a particular depth. In effect, the hydrostatic pressure is the average pressure base line about which the hydrodynamic pressure variations fluctuate.